Docosahexaenoic acid (22: 6 Δ4, 7, 10, 13, 16, 19, referred to as DHA), commonly called “brain gold”, belongs to ω-3 polyunsaturated fatty acid and has the physiological functions of brain fitness, prevention of senile dementia, improvement of eyesight, prevention and treatment of myopia, prevention of hypertension, arteriosclerosis and arthritis, treatment of cancers and etc. Therefore, DHA has broad market prospects as a new generation of functional health factor. With increasing market demands, the requirement for DHA quality becomes higher and higher, the direction of research on producing DHA has been gradually developed towards biosynthesis, and DHA from traditional fish oil has been gradually replaced by algal oil DHA. The algal oil DHA refers to oil containing DHA and other long-chain polyunsaturated fatty acids obtained by microbial fermentation, wherein DHA-producing fungi mainly comprise Schizochytrium, Thraustochytrium, Crypthecodinium and etc.
Schizochytrium, also named as Schizochytrium limacinum, is unicellular, spherical and one kind of algae-like marine fungi, which belongs to Thraustochytriaceae, Saprolegniales, Oomycetes, Eumycota. A large amount of active substances which are useful to human bodies are accumulated within the cell, such as oil, pigments (carotenoids, lutein and astaxanthin), squalene and etc., wherein the content of DHA in total fatty acids is up to 35%-45%, and more than 90% of oil in the cell exists in neutral oil form, i.e. triglyceride, which is easily absorbed by human body. Thereby Schizochytrium is an ideal strain for producing DHA.
Chinese patent CN101575584 discloses Schizochytrium sp. and method for producing DHA oil by using same. The method optimizes a strain fermentation medium from the perspectives of osmotic pressure and element supply, combines a fed-batch strategy to achieve high-density fermentation of Schizochytrium, and adopts a traditional fermentation method, which comprises the steps of inoculating seeds saved in glycerine into a shake flask to activate, preparing a primary seed solution, preparing a secondary seed solution in the shake flask, culturing in a primary seed tank, culturing in a secondary seed tank and fermenting, thus the final dry cell weight reaches 70 g/L, the oil content reaches 31.5 g/L, and the DHA accounts for more than 35% of the total fatty acids content.
Chinese patent CN101519676 discloses a fermentation production process for production of DHA by using a fermentation culture medium containing trace elements, which includes using traditional method to culture Schizochytrium strain, namely slant activation culture, then enlarging culture by transferring into a shake flask, and performing primary enlarging culture to obtain a seed solution for fermentation; or culturing with slant activation, than enlarging culture by transferring into a shake flask, and performing primary enlarging culture and secondary enlarging culture to obtain a seed solution for fermentation, and by fermenting in a 10T tank for 95 h, the biomass is 53 g/L, the oil content is 72%, and the content of DHA is 50%.
However, the above mentioned traditional seed culture and fermentation method involves many fermentation stages, the process is very complicated, the culture period is long and the investment cost is high. Thus, it is urgent in the art to improve seed preparation method, simplify seed enlarging process, and reduce the production cost, to benefit the production of DHA.